When cracking palm nuts, capuchin monkeys prefer using stones that have already been used, a study shows. Researchers found that when only unused stones were available, monkeys picked softer stones to put the nut onto and harder stones to hit it with. However, when stones were worn from previous nut-cracking use, the monkeys were more likely to pick used stones to put the nut onto, regardless of hardness. The researchers say this may provide insights into how tool use evolved in humans, as peopl